DC-DC converter for fuel cell application using hybrid inductor core material

ABSTRACT

An inductor that includes end core pieces and a plurality of center block core pieces between the end core pieces that define a plurality of gaps. The end core pieces are made of an amorphous alloy to provide good magnetic field properties and the center core pieces are stamped magnetic sheets forming a laminate structure to provide ease of manufacturability. In one non-limiting embodiment, the end core pieces are an amorphous iron alloy and the center core pieces are stamped silicon-iron.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to an inductor including multiplecenter core pieces providing multiple gaps and, more particularly, to aninductor including end core pieces and multiple center core piecesproviding multiple gaps where the end core pieces are made of anamorphous alloy to provide good magnetic properties and the center corepieces are a stamped plate laminate structure to provide ease ofmanufacturability.

2. Discussion of the Related Art

Hydrogen is a very attractive fuel because it is clean and can be usedto efficiently produce electricity in a fuel cell. A hydrogen fuel cellis an electrochemical device that includes an anode and a cathode withan electrolyte therebetween. The anode receives hydrogen gas and thecathode receives oxygen or air. The hydrogen gas is dissociated in theanode to generate free hydrogen protons and electrons. The hydrogenprotons pass through the electrolyte to the cathode. The hydrogenprotons react with the oxygen and the electrons in the cathode togenerate water. The electrons from the anode cannot pass through theelectrolyte, and thus are directed through a load to perform work beforebeing sent to the cathode.

Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell forvehicles. The PEMFC generally includes a solid polymer electrolyteproton conducting membrane, such as a perfluorosulfonic acid membrane.The anode and cathode typically include finely divided catalyticparticles, usually platinum (Pt), supported on carbon particles andmixed with an ionomer. The catalytic mixture is deposited on opposingsides of the membrane. The combination of the anode catalytic mixture,the cathode catalytic mixture and the membrane define a membraneelectrode assembly (MEA). MEAs are relatively expensive to manufactureand require certain conditions for effective operation.

Several fuel cells are typically combined in a fuel cell stack togenerate the desired power. For example, a typical fuel cell stack for avehicle may have two hundred or more stacked fuel cells. The fuel cellstack receives a cathode input gas, typically a flow of air forcedthrough the stack by a compressor. Not all of the oxygen is consumed bythe stack and some of the air is output as a cathode exhaust gas thatmay include water as a stack by-product. The fuel cell stack alsoreceives an anode hydrogen input gas that flows into the anode side ofthe stack.

The fuel cell stack includes a series of bipolar plates positionedbetween the several MEAs in the stack, where the bipolar plates and theMEAs are positioned between two end plates. The bipolar plates includean anode side and a cathode side for adjacent fuel cells in the stack.Anode gas flow channels are provided on the anode side of the bipolarplates that allow the anode reactant gas to flow to the respective MEA.Cathode gas flow channels are provided on the cathode side of thebipolar plates that allow the cathode reactant gas to flow to therespective MEA. One end plate includes anode gas flow channels, and theother end plate includes cathode gas flow channels. The bipolar platesand end plates are made of a conductive material, such as stainlesssteel or a conductive composite. The end plates conduct the electricitygenerated by the fuel cells out of the stack.

Most fuel cell vehicles are hybrid vehicles that employ a supplementalpower source or rechargeable electrical energy storage device (RESS) inaddition to the fuel cell stack, such as a high voltage DC battery, asuper-capacitor or an ultracapacitor. The RESS provides supplementalpower for the various vehicle auxiliary loads, for system start-up andduring high power demands when the fuel cell stack is unable to providethe desired power. The fuel cell stack provides power to an electrictraction motor through a DC high voltage bus line for vehicle operation.The RESS provides supplemental power to the voltage bus line duringthose times when additional power is needed beyond what the stack canprovide, such as during heavy acceleration. For example, the fuel cellstack may provide 70 kW of power. However, vehicle acceleration mayrequire 100 kW of power. The fuel cell stack is used to recharge RESS atthose times when the fuel cell stack is able to provide the system powerdemand. The generator power available from the traction motor duringregenerative braking is also used to recharge the RESS.

In the hybrid vehicle discussed above, a boost DC/DC converter issometimes employed to match the lower voltage fuel cell stack to thehigher voltage RESS. DC/DC converters often employ a multi-phase arrayof inductors that provide the task of increasing the DC voltage. Atypical inductor in the inductor array includes a magnetic core, such asan iron core, where a gap is provided between core pieces. Isolatedmetal windings are wrapped around the core as a coil and a currentpropagating through the windings generates a magnetic flux in the coreand the gap. Suitable switching is provided to switch the voltageapplied to the windings so that the magnetic flux change increases thevoltage potential at the output of the converter.

There has been an effort in the industry to reduce the size, weight andcost of DC/DC converters in fuel cell systems for vehicles, and increasetheir reliability and efficiency.

SUMMARY OF THE INVENTION

In accordance with the teachings of the present invention, an inductoris disclosed that includes end core pieces and a plurality of centerblock core pieces between the end core pieces that define a plurality ofgaps. The end core pieces are made of, for example, an amorphous alloyto provide good magnetic properties and the center core pieces are, forexample, stamped electrical steel sheets forming a laminate structure toprovide ease of manufacturability. In one non-limiting embodiment, theend core pieces are an amorphous iron alloy and the center core piecesare stamped silicon-iron.

Additional features of the present invention will become apparent fromthe following description and appended claims, taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a fuel cell system including aDC/DC converter having an inductor array, according to an embodiment ofthe present invention;

FIG. 2 is a cross-sectional view of an inductor core including U-shapedinductor pieces defining a gap therebetween;

FIG. 3 is a cross-sectional view of an inductor core including centerinductor pieces defining a plurality of gaps;

FIG. 4 is a cross-sectional view of an inductor array including threespaced apart inductors;

FIG. 5 is a cross-sectional view of an inductor array including threeinductors sharing common core pieces and a flux path, according to anembodiment of the present invention; and

FIG. 6 is a cross-sectional view of an inductor array including threeinductors having shared core pieces and flux path and including multiplecenter core pieces defining multiple core gaps, according to anotherembodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following discussion of the embodiments of the invention directed toan inductor including end core pieces made of an amorphous alloy andcenter core pieces providing multiple gaps formed of stamped metalsheets is merely exemplary in nature, and is in no way intended to limitthe invention or its applications or uses. For example, the inductor ofthe invention has particular application for an inductor in an inductorarray for a DC/DC boost converter in a fuel cell system. However, aswill be appreciated by those skilled in the art, the inductor of theinvention may have application for other systems.

FIG. 1 is a schematic block diagram of a fuel cell system 10 including asystem fuel cell stack 12 and a rechargeable electrical energy system(RESS) 14 both electrically coupled to high voltage bus lines 16 a and16 b. The RESS 14 can be any suitable DC power source for the purposesdescribed herein, such as a battery or an ultracapacitor (also referredto as a super-capacitor), etc. Various electrical components areelectrically coupled to the high voltage bus lines 16 a and 16 b, suchas an electric traction motor 22 that propels the vehicle, and othervehicle and system loads 24, such as a cathode side air compressor.

A simplified DC/DC converter 26 is provided to boost the fuel cell stackvoltage to the higher voltage of the bus lines 16 a and 16 b. The DC/DCconverter 26 includes an inductor array 28 and a controlled switchingnetwork 30, where the switching network 30 switches inductors 38 in theinductor array 28 from high voltage ground to high voltage output inorder to boost the fuel cell stack voltage in a manner that is wellunderstood to those skilled in the art. This conversion method iscommonly known as boost conversion. Boost converters can be built usinga single inductor or by using a plurality of time shifted switchinginductors in parallel boost stages to smooth the switching rippleeffects. The DC/DC converter 26 provides a 3-phase boost converterdesign for this purpose.

FIG. 2 is a cross-sectional view of a core 40 of an inductor for a DC/DCconverter of the type discussed above. The core 40 includes a firstU-shaped core piece 42 and a second U-shaped core piece 44 spaced apartfrom each other to define a gap 46 therebetween. Conductor coils,discussed below, are wound around the legs of the U-shaped core pieces42 and 44, and when current flows through the coils, a magnetic flux iscreated in the core 40 and the gap 46 that provides the energy for thevoltage conversion. Traditional core pieces for inductors of this typeare stamped sheets of a magnetic material, such as iron, that arelaminated together to provide the desired core thickness.

Because the gap 46 defines the amount of magnetic energy that will bestored in the inductor, it is often desirable to make the gap 46 wide.However, as the width of the gap 46 increases, a fringing flux at thecorners of the U-shaped pieces 42 and 44 increases where the magneticfield lines extend away from the core 40. The fringing flux creates eddycurrent losses in the core material and windings and adds an efficiencyloss to the system.

In order to overcome the drawback of fringing flux, and still providesignificant gap space for high power applications, it is known in theart to provide more than one gap in an inductor core. FIG. 3 is across-sectional view of an inductor core 50 including U-shaped endpieces 52 and 54 and four center block pieces 56, 58, 60 and 62positioned therebetween. The U-shaped pieces 52 and 54 and the centerpieces 56, 58, 60 and 62 are spaced apart to define three narrow gaps64, 66 and 68 at the center of the core 50, as shown. Therefore, theoverall gap size can be increased to provide an increase in magneticstored energy without the detrimental fringing flux that occurred as aresult of wide gaps.

Traditional cores for inductors of this type are typically laminatedpieces of iron or other ferromagnetic material that provide goodmagnetic field properties. In order to reduce the losses in the corematerial, it is known in the art to use an amorphous alloy material thathas superior magnetic properties to regular iron. However, the amorphousalloy cannot economically be manufactured into as small of a size as thepieces 56, 58, 60 and 62 would need to be for a fuel cell system boostconverter application using standard low cost manufacturing andsegmenting cutting processes because of the materials brittleness.

Various amorphous alloys are known in the art for magnetic cores in aninductor. These amorphous alloys include alloys made from iron, nickel,cobalt, etc. The core material needs to be manufactured in a highlycontrolled manner to produce an amorphous structure, i.e.,non-crystalline, that provides the desired magnetic properties. In oneknown technique, a molten alloy is cast through a nozzle onto a rotatingcooled copper wheel, which causes a thin layer of the alloy toimmediately solidify and peel off of the wheel in a thin endless ribbon.The solidification process is so fast that the alloy cannot formcrystals. These ribbons have excellent magnetic properties, but becausethey are thin and have low electrical conductivity due to theiramorphous structure, eddy current losses in the material is extremelysmall. Therefore, those materials can be used in high power density,high frequency applications where regular iron core laminates wouldoverheat from the losses. The ribbons are wound on a mandrel with tracesof glue in between to get a solid O-shaped core. The O-shaped core istypically cut in the middle to obtain the two U-shaped end piecesdiscussed herein.

The present invention proposes a hybrid core for the core 50 thatincludes both the amorphous alloy for the U-shaped pieces 52 and 54, andthe traditional stamped metal sheet cores for the center pieces 56, 58,60 and 62. Thus, the center pieces 56, 58, 60 and 62 are stamped piecesthat have been laminated together by known manufacturing processessuitable for the size of the sections 56, 58, 60 and 62, and combinedwith the amorphous alloy pieces 52 and 54 that complete the magneticflux path. Various materials are known in the art that are suitable forthe core material of an inductor and are suitable for the known stampingprocesses of sheet materials that are formed together into laminates.For example, silicon-iron materials can be used, where the iron providesthe desired magnetic properties and the silicon improves theperformance, generally by reducing eddy current losses.

The losses of the core 50 are determined by the majority of thematerial, which will still be the amorphous metal, while the smaller gapforming pieces have relatively high specific losses, but in a very smallvolume so that they do not contribute significantly to the overalllosses. The magnetic properties, especially saturation flux density, ofamorphous iron and regular stamped sheet iron are sufficiently close sothat the core performance is not limited by either of the materials.Combining the advantages of the amorphous metal with the advantages ofthe regular stamped sheet cores allows smaller power conversioninductors to be built with lower cost, size, weight and magnetic losses.

FIG. 4 is a cross-sectional view of an inductor array 70 suitable to beused as the inductor array 28 in the system 10 of the type that is knownin the art. The inductor array 70 includes a housing structure 72including mounting holes 74 for mounting the array 70 at a suitablelocation. The inductor array 70 has three closely spaced inductors 76each including a magnetic core 40 having U-shaped end pieces 78 and 80defining a gap 82 therebetween. Each inductor 76 includes two windings84 and 86 that are wound around leg portions of the end pieces 78 and80, as shown, and through a central opening 88 between the end pieces 78and 80. The coils 84 and 86 would be electrically coupled together,either in series or in parallel. There are three of the inductors 76 inthe array 70 to provide a smooth transition for the voltage conversion.Particularly, the inductors 76 are switched into and out of the circuitusing the switching network 30 with a defined phase relationship so thatthe voltage conversion is a smooth transition for switching of the coils84 and 86.

FIG. 5 is a cross-sectional view of an inductor array 100 including ahousing structure 102 and mounting holes 104, according to an embodimentof the present invention. The inductor array 100 includes threeinductors 106, 108 and 110. Although three inductors are provided in theinductor array 100 for this design, this is merely by example as acompromise in complexity verses higher inductor phase count smoothingbenefits. This part of the invention can be provided for an inductorarray including inductors of two or more. The inductor 106 includescoils 112 and 114, the inductor 108 includes coils 116 and 118, and theinductor 110 includes coils 120 and 122. According to the invention,adjacent inductors 106, 108 or 110 include a shared core piece.Particularly, the inductors 106 and 108 share a core piece 124 and theinductors 108 and 110 share a core piece 126. The inductor 106 includesa U-shaped end core piece 128 opposite to the core piece 124 and theinductor 110 includes a U-shaped end piece 130 opposite to the corepiece 126. A gap 132 is provided between the core pieces 124 and 128 forthe inductor 106, a gap 134 is provided between the core pieces 124 and126 for the inductor 108, and a gap 136 is provided between the corepieces 126 and 130 for the inductor 110.

As can be seen, magnetic flux lines 90 from the inductors 106 and 108travel in opposite directions in the core piece 124, and magnetic fluxlines 90 of the inductors 108 and 110 travel in opposite directions inthe core piece 126. Thus, as the strength of the flux lines 90 increasesand decreases in response to the switching phase relationship betweenthe inductors, there is at least a partial cancellation of the fluxlines 90 propagating in opposite directions in the common flux partsegment.

Because the inductors 106 and 108 share a core piece and the inductors108 and 110 share a core piece, the amount of core material used in thearray 100 can be reduced over the amount of core material used in thearray 70 for the same size inductor array. Therefore, the size andweight of the inductor array 100 can be reduced over the inductor array70. For example, the length of the array 100 can be about 215 mm for oneparticular boost converter application, which is a 30% improvement overthe length of the array 70. Further, by combining the core pieces asdescribed herein, the electrical losses of the inductor array 100 can bereduced from the electrical losses of the inductor array 70.

The core pieces 124 and 126 have an indented portion 138 at theircenters halfway between the gaps in the inductors, such as would beformed by two back-to-back U-shaped members. As the flux cancels atleast partly, the common core pieces can even be made such that they aresmaller in cross-section than would be required for an individual corepiece. In an alternate embodiment, those indented portions could besignificantly smaller to further reduce the size of the core pieces 124and 126, and thus the cost and weight of the array 100.

FIG. 6 is a cross-sectional view of an inductor array 140 thatincorporates the advantages of the design of the core 50 shown in FIG. 3and the design of the cores in the inductor array 100 shown in FIG. 5,according to another embodiment of the present invention. Particularly,the inductor array 140 includes three inductors 142, 144 and 146. Theinductors 142 and 144 share a core piece 148 similar to the core piece124, and the inductors 144 and 146 share a core piece 150 similar to thecore piece 126. The inductor 142 includes end core piece 162 and theinductor 146 includes end core piece 164. Further, the inductor 142includes center core pieces 152 that define three gaps 154, the inductor144 includes center core pieces 156 that define three gaps 158 and theinductor 146 includes center core pieces 160 that define three gaps 162.As above, the core pieces 148 and 150 can be an amorphous alloy and thecenter core pieces 152, 156 and 160 can be stamped metal core piecesthat form a laminate and allow for ease of manufacturability.

The foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. One skilled in the art willreadily recognize from such discussion and from the accompanyingdrawings and claims that various changes, modifications and variationscan be made therein without departing from the spirit and scope of theinvention as defined in the following claims.

1. An inductor array comprising a plurality of inductors, each of theinductors in the inductor array including a plurality of core piecesincluding opposing end core pieces that are spaced apart from eachother, wherein adjacent inductors share a single core piece and a commonflux path, said plurality of core pieces further including a pluralityof center block core pieces defining a plurality of gaps at the centerof each inductor, wherein the end core pieces are made of an amorphousalloy and the center block core pieces and single core pieces arestamped metal sheets formed as a laminate.
 2. The inductor arrayaccording to claim 1 wherein the shared core pieces are configured asback-to-back U-shaped core pieces with an indented center region.
 3. Theinductor array according to claim 1 wherein the amorphous alloy is aniron alloy.
 4. The inductor according to claim 1 wherein the opposingend core pieces are U-shaped end core pieces defining a central openingthrough which coils are wound.
 5. The inductor array according to claim1 wherein the stamped metal center core pieces and single core piecesare stamped silicon-iron core pieces.
 6. The inductor array according toclaim 1 wherein the plurality of inductors includes three inductors. 7.The inductor array according to claim 1 wherein the inductor array ispart of a DC/DC boost converter.
 8. The inductor array according toclaim 7 wherein the DC/DC boost converter is part of a fuel cell systemand is electrically coupled to a fuel cell stack.
 9. An inductor arrayfor a DC/DC boost converter, said inductor array including threeinductors where each of the inductors includes a plurality of corepieces including opposing end core pieces that are spaced apart fromeach other, wherein adjacent inductors share a single core piece and acommon flux path, said shared single core pieces being configured asback-to-back U-shaped core pieces with an indented center region, saidplurality of core pieces further including a plurality of center blockpieces, wherein the plurality of center block pieces are substantiallyparallel and positioned to form a plurality of gaps at the center ofeach inductor, wherein the end core pieces are made of an amorphousalloy and the center block core pieces and shared single core pieces arestamped metal sheets.
 10. The inductor array according to claim 9wherein the amorphous alloy is an iron alloy.
 11. The inductor arrayaccording to claim 9 wherein the stamped metal center core pieces andshared single core pieces are stamped silicon-iron core pieces.
 12. Theinductor array according to claim 9 wherein the DC/DC boost converter ispart of a fuel cell system and is electrically coupled to a fuel cellstack.